1. Field of the Invention
The present invention relates to a withdrawal device for films manufactured in a blow-molding process.
2. Description of the Prior Art
A withdrawal device for films manufactured in the blow-molding process is known, for instance from the patent specification DE 195 22 318 A1. After the film, for instance, a plastic tubular film emerging from an extruder has been laid flat, it is guided by means of air turning bars and deflecting rollers that as a rule reverse against one another and usually rotate around a common axis. After running through the withdrawal device the film is fed to a winding device on which it is wound into a roll for the purpose of collecting it and transporting it away easily.
The air turning bars used in withdrawal devices of such type were frequently executed in a way allowing compressed air to flow through them, which can emerge through drilled holes distributed across the casing of the air turning bars, in order for an insulating air cushion to be formed between the surface of the air turning bar and the plastic tubular film drawn over it enabling the film to be guided in an almost contact-free manner. In particular, in processing films that range from adhesive to sticky, it is particularly desirable to carry the film with large insulating air cushions as this causes the film guided over the air turning bar to float on the entire width of the air turning bar.
One problem in the known air turning bars is the uneven thickness of the air cushion over the axial width of the bar and/or the uneven force acting on the film due to the compressed air. Air can escape from the side regions of the film, thus reducing the amount of compressed air in these areas. The side regions of the film wrap around the turning bar more tightly than the center regions of the film. Consequently, in its center region the film exhibits a certain sag and can no longer be guided in a flat form. The film in this center region is prone to the formation of wrinkles.
Therefore an air turning bar is suggested in the patent specification DE 9418255 U1 that is molded in a convex form and thus provides an even web tension over the entire width of the film. The air turning bar introduced there tapers in the direction of its axial side regions. However, it turns out that the films that are guided in this manner cannot be wound up into a perfect roll.
Therefore the patent specification DE 44 40 647 A1 suggests an air turning bar with a concave shape that accommodates the variable thickness of the air cushion while at the same time enabling the film to guide in a flat manner. In the center portion of the air turning bar the film is guided with a greater distance from the turning bar than in the end portions of the turning bar. Due to the lesser distance from the turning bar in the end portions, the forces that are distributed over the width of the film and that act on the film due to the compressed air exhibit lesser variations.
In practice it has been proven even in case of this embodiment of the air turning bar that the air cushion between the turning bar and the film does not apply a sufficiently even force on the film. Consequently the film cannot be guided with an even web tension across its width. In particular, such an air turning bar can be adjusted only to one film width. However, in present times one blowing head of the extruder is used to manufacture films of differing widths.